1. Field of the invention
The invention relates to data processing apparatus for recording and/or replaying digital data (e.g. video or image data) with a plurality of heads. The invention finds particular application in the recording of digital video data.
2. Description of the Prior Art
Given the high information densities and stringent requirements under which digital video tape recording systems operate, it is not practical to design systems in which no recording and/or playback errors are expected to occur. Typical of the sorts of problems which occur are the loss of data due to a recording of playback head becoming clogged with dirt and/or recording material from the tape, inhomogeneities in the recording layer(s) on the tape and scratches in the tape. Accordingly, known digital video tape recording systems include apparatus and methods for coping with errors which occur during the recording and/or playback of video information as part of the overall operation of such systems.
As a first level of protection against such errors, error correction codes are included with the recorded video data. If the error is a minor one then it is often possible to uniquely identify the particular piece or pieces of information that are in error and what data they should be replaced with. If the error is too severe, then such error correction cannot cope and then reliance is made upon error concealment techniques to reduce the perceivable effect of the error.
At the data rate required for recording digital video signals it is necessary to use a plurality of recording heads recording several tracks fop one video field. As a consequence of this and in order to facilitate error concealment, it is known to sub-sample the image data into a number of different data processing and recording channels. In this way, if an error occurs in one channel, then there will be data from the remaining channels surrounding the missing data points from the defective channel. For each erroneous pixel, a replacement pixel value can be interpolated from the surrounding pixel values from the other channels within the same field or frame or alternatively from the corresponding pixel position in preceding and/or following video fields or frames. Whilst detail is still lost from the image by such errors, the overall effect of such error concealment is to make the error less immediately perceivable. An example of digital video tape recorder apparatus employing such an approach to error handling is described in GB-A-2 140 189. In this known apparatus, with a recording head assembly having 2 n heads, where n is 1, 2 or 3, a demultiplexer demultiplexes video samples of an incoming digital television signal sample-by-sample into 2 n channels for supply to the 2 n recording heads and a switching arrangement for switching the connections between the channels and the heads line-by-line and possibly also field-by-field or frame-by-frame of the television signal. Although GB-A-2 140 189 mentions the idea of switching the head allocation field-by-field or frame-by-frame, i.e. a temporal demultiplexing of the video signals it does not describe a detailed implementation of this. In practice a sample-by-sample, or spatial demultiplexing of the video signals has to date been found sufficient.
GB-A-2 140 189 describes the demultiplexing of video data into four channels for supply to each of four heads A, B, C and D. A stream of video pixels for a video field is received as a stream of pixels, pixel-by-pixel from left to right within a scan line and line-by-line. The demultiplexing is applied in a cyclical manner so that successively received pixels are applied to respective ones of the heads A, B, C and D. To facilitate concealment of errors each pixel is arranged to be surrounded by eight pixels not processed by the same head, switching occurs between the heads A and C and between the heads B and D on a line-by-line basis. The result of the demultiplexing operations described in GB-A-2 140 189 is illustrated in FIG. 23. It can be seen that each line of pixels contains the sequence A, B, C, D, A, B, C, D, and so on, with, however, the sequence displaced in alternate lines by two pixel positions within that line. This simple structure always ensures that a pixel is surrounded by pixels from the other three heads. The demultiplexing strategy described in GB-A-2 140 189 has been found to be satisfactory in most cases where digital pixel samples are recorded directly on tape.
In view of the high information densities involved in image data processing, particularly as image definition increases, it is desirable that some form of data compression be performed upon the image data before it is recorded. One set of techniques for achieving such data compression involves the decorrelation of the image data from the spatial domain into a transform domain. Once decorrelated into the transform domain, the redundancy within the image data can be better exploited to yield efficient compression. The data is stored or transmitted as an encoded version of the image in the transform domain. Decorrelation is a technique for redistributing the energy of the signal into different frequency bands where they are more suited to the application of efficient coding methods.
It has been proposed to apply an analogous approach to that described in GB-A-2 140 189 with data compression by sub-sampling video data into the transform domain and interpolating any erroneous parts in the reproduced transform domain data from the immediately adjacent parts of the transform domain data. It was proposed to apply spatial demultiplexing of the video samples only, this having proved to be adequate in prior systems. It was proposed to lay down data on magnetic tape in data blocks, each of which contains a transformed, encoded and compressed version of a set of sub-sampled data from the original input image data. If errors in reproducing the data (e.g. drop outs) occur, then these are most likely to affect only individual data blocks with a resultant loss of only one sub-sampled part of the original input image data from a particular region of the image in the spatial domain. In this way, it was intended that other sub-sampled parts of the input image data could be used to provide effective error concealment.
However, it has been found that the performance of the spatial demultiplexing strategy of GB-A-2 140 189 was much less effective when applied in the transform domain than in the prior art. Also although GB-A-2 140 189 does suggest the idea of temporally demultiplexing the video data, it does not teach a systematic approach to carrying this out.
Our co-pending UK patent application GB 9214299.1, filed on the same date as this application, relates to a multi-head recording system using spatial and temporal demultiplexing of data. The co-pending patent application uses a recording head mechanism with multiple recording heads organised in groups, where each group comprises a plurality of recording heads connected in common to a head channel. It is relatively easy to ensure the data is recorded (and subsequently replayed) in a synchronised manner when this occurs within a single data channel. However, synchronisation of data stored by different head channels is a much more difficult matter. The object of this invention is to enable the efficient temporal multiplexing of data recorded in a spatially and temporally demultiplexed manner by a multi-head recording system having multiple heads organised in groups where each group comprises a plurality of heads connected in common to a head channel.